When the first coronavirus vaccines were administered last week, many front-line health-care workers across the U.S. posted celebratory photos on social media. In what was meant to be a triumphant moment, some found themselves the targets of skeptical or even scornful comments from people opposed to vaccines.

A nurse on Twitter with the handle @saruhhdanae shared a picture of her getting the Pfizer shot, in a post with eight exclamation points. Soon, amid notes of congratulations and appreciation, came responses from people declaring they would be too afraid to take it, predicting terrible side effects, or worse. “I made a choice for myself and all kinds of people are predicting my death,” she said in a follow-up tweet, noting that she was still happy to be vaccinated.

Now that coronavirus vaccines have been created and distributed in record time, the first people to get it have become de facto online ambassadors to reassure the public that it’s safe. They’re up against the anti-vaccine movement, which for years has propagated rumors about the dangers of common, safe vaccines in Facebook groups and on Instagram influencer accounts. As U.S. states figure out how to roll out the inoculations as doses trickle in, some people are wondering whether to get it at all, often because they’re scrolling in online communities where emotional anecdotes spread faster than accepted science.

Battles in the online comments are wearing down already stressed nurses, said Dan Weberg, head of clinical innovation at Trusted Health, which connects travel nurses with hospitals.

“After getting beat up for 10 months in the pandemic, now they’re getting beat up on social media for making a choice that could save hundreds of thousands of lives,” Weberg said.

Social media platforms like Facebook Inc., Twitter Inc. and YouTube are starting to reckon with the role they’ve played in the persistent popularity of vaccine skepticism. Twitter said that starting this week, it will remove any content that spreads vaccine-related conspiracies or false and widely debunked claims about the shot’s effects. By early next year, the company may label incomplete or out-of-context vaccine information with a link to information from health authorities.

Facebook, which has been attempting to keep anti-vaccination groups out of its recommendations to users, said it will start looking at ways to encourage health-care workers who are sharing the positive news of their shots. “As that trend continues, we’re actively looking for ways to promote and support it,” the company said. It’s also updated a covid-19 information center with vaccine facts.

In first-shot posts that have flooded Instagram, Facebook, TikTok and Twitter, health-care workers have sometimes anticipated the blowback in their choice of language. “I hope to show my family, friends, and patients that the vaccine is safe, effective, and crucial to our lives returning to normal,” tweeted Madeline DiLorenzo, a resident at Boston Medical Center.

“I’m shocked my friends are asking if I’d get the #COVIDVaccine…. HELL YEAH!!” Nasrien Ibrahim, a cardiologist in Boston, said on Instagram. “I’m a clinician and researcher. I believe in science. What do you think happened to polio?”

Even in Facebook groups for medical professionals, the vaccine has sparked intense debate. Kaithlyn Rojas, who works with intubated COVID-19 patients at Highland Hospital in Oakland, California, said she can’t believe what she’s seeing in nursing groups on Facebook.

“Someone will comment about a reason they won’t get the vaccine, and there will be a rebuttal, going back and forth between the scientific evidence and rumor,” Rojas said. “There are a lot of opinions out there that are being used or sourced as factual information, just like in the elections we just had.” In an October survey by the American Nurses Federation, 37% of nurses said they weren’t confident the vaccines would be safe and effective.

The Center for Countering Digital Hate found that the anti-vaccination movement has grown online despite platforms’ efforts to curtail it, thanks to influencers with large followings and strategic attempts by those leaders to recruit vaccine-hesitant people, especially mothers and ethnic minorities. The group asked volunteers to report vaccine misinformation to social media companies in the fall; out of 912 posts flagged as problematic, less than 5% were removed.

The coronavirus vaccine could make it easy for their message to reach a broader audience, Imran Ahmed, CEO of the Center for Countering Digital Hate, said in the group’s recent report. “Anti-vaxxers view the coronavirus vaccine as an opportunity to create long-lasting distrust in the effectiveness, safety, and necessity for vaccination,” he said. “Unless urgent action is taken, they may succeed.”

Ashley Sayles, a pediatric nurse practitioner in Baltimore, said she sees extreme hesitancy around vaccines among mothers, especially in the Black community. She tries to debunk medical misinformation via her Instagram, where she has 25,000 followers. She has chosen not to post yet about the coronavirus vaccine because her patients aren’t eligible to receive it. By the time they can, she’ll have more data to add to the argument.

“I know for a fact I’ll have a hard time convincing families with this COVID vaccine,” she said. “I can barely get them to take the flu shot that’s been around this long.”

Either way, she’s not expecting help from social media companies: “With the way that the algorithms work, if you want to see vaccine skepticism, you’re gonna see it.”